The invention relates to a method of producing improved fluff pulps. The invention is also concerned with the use of the improved fluff pulp in disposable hygiene products and air-laid products.
In the invention, the defibration properties of fluff pulp are improved by enzyme treatment.
The worldwide production of fluff pulps amounts to about 2.5 million tons. Softwood with long fibres only is used in the production. More than 90% of the pulps are fully bleached chemical pulps, of which more than 90% are sulphate pulps. The proportion of CTMP (chemi-thermomechanical pulps) is less than 10%. They are usually peroxide bleached to a degree of brightness of 70 to 80% ISO. TMP (thermomechanical pulp) and groundwood have also been used as fluff pulp though not to any greater degree.
The cooking and bleaching of chemical fluff pulps do not significantly differ from the production of paper pulps. As far as CTM pulpS are concerned, their freeness (drainability) is markedly higher than that of paper pulps, ranging from 500 to 700 ml CSF.
The web formation, wet pressing, and drying of fluff pulps differ greatly from the production of paper pulp. Moreover, these process steps are more critical for the quality and processability of the final product. More than 95% of fluff pulps are delivered to the customer in rolls in which the moisture of the pulp varies between 5 and 10%.
Fluff pulps are used as raw material in the absorbent layers of disposable hygiene products, such as baby diapers, sanitary napkins, panty shields, incontinence pads, and absorbent hospital sheets, as such or with superabsorbents and/or synthetical fibres. More than 80% of the pulps are used in baby diapers.
The most demanding application of fluff pulps is in the so called air-laid products, such as air-laid papers used, e.g., in serving utensils, various towel applications in homes, in the industry and in hospitals. Internationally, the demand of pulp for these products amounts to 70,000 80,000 t/a, and the yearly growth is more than 10%. (Chemical pulp only is used.) Some of the products are colored, whereby either the chemical pulp is colored or the coloring is carried out in connection with the air-laying step.
The dry defibration of pulp (mostly chemical pulp or CTMP) is one of the most important process steps in the production of both napkins and air-laid products. A chemical pulp web (1 to 3 superimposed webs) is thereby led from a roll into a shredder, usually a hammer mill but pin mills and disc refiners can be used as well. In some cases, it is possible to carry out the shredding in two stages, e.g., with a disc refiner and a hammer mill. When using bale pulps, the pulp is first cut into strips which are introduced into the shredder through a separate dosing device. In the shredder, the pulp web is passed between rotating shredding means, such as hammers, and counter blades, positioned at a distance of a few millimeters from each other. This treatment aims at detaching wood fibres contained in the pulp (moisture typically 7%) from each other as undamaged as possible. The shredding process can be assessed by measuring, e.g., decrease in the fibre length, fines or dust formation, and the knot content, i.e., the amount of knots remaining in the pulp (fibre bundles, knots, etc.), and the defibration or shredding energy. The amount of energy required for the production of hygiene products varies within very wide limits, ranging from 80 to 250 MJ per one ton of pulp. The knot content varies in different hygiene products typically from 5 to 30% measured by the SCAN knot tester (SCAN-CM 37:85). Instead, in air-laid products, the knot content should be under 1 %, i.e., the product may contain very little if any knots. The object is to keep the knot level as low as possible with the lowest possible energy consumption while maintaining the fibres undamaged. Excessive consumption of energy does not only result in economical losses but also in other drawbacks such as decreasing fibre lengths, dust formation, and static charging of fibres, which in turn hampers the formation of an even web.
The ratio between the knot content and the energy needed depends mainly on the pulp production process and the density of the pulp web or sheet. Typically, sulphate pulps are "harder", that is, they require more energy than sulphite or CTM pulps. Also, an increase in the density of pulp web caused by wet pressing always increases the energy demand.
With chemical and CTM pulps, attempts have been made to improve the shredding properties of fluff pulp (shredding energy/knot content) by treating the pulp prior to the drying step with debonding chemicals. These, however, always degrade the absorbency of the pulp, and most hygiene product manufacturers do not accept the application of debonding chemicals, wherefore the pulp manufacturers can rely on the debonding treatments only very limitedly.
The most effective way of improving shredding properties is to reduce the density of the pulp web, that is, to prevent the formation of hydrogen bonds between fibres. This, however, always results in a decrease in the drying capacity, which means reduced profitability, because in most fluff pulp factories, it is the drying step that limits the production capacity.
CA patent specification 1,206,305 discloses one solution for improving the shredding properties of fluff pulp. Chemical pulp in the form of an aqueous slurry is therein treated with gaseous ammonia under pressure, whereby the obtained fluff pulp has improved shredding properties. The final product is also recited to exhibit improved absorbency.
However, methods known from the prior art do not offer a satisfactory solution to problems associated with the shredding of fluff pulp.
Examples of enzyme treatment in the production of pulp can be found in the prior art. FR patent specification No. 2,557,894 describes a method of treating chemical pulp with xylanase enzyme with the purpose of shortening the refining time. CA patent specification No. 758,488 is concerned with a method of improving the refining properties of pulp by a cellulase/pectinase/lipase enzyme treatment. FR patent specification No. 2,571,738, in turn, discloses a method in which pulp is provided with special properties by cellulase treatment JP patent specification No. 60,126,395 discloses a method of improving the refining process by enzyme addition.
JP patent specification No. 59,009,299 describes a method in which alkaline cellulase in combination with a surfactant is added to a deinking process for improved deinking.
JP patent application No. 63,059,494 discloses a method of improving the brightness of reclaimed paper pulp by means of alkaline cellulase.
FR patent application No. 8,613,208 describes a method of improving the properties of recycled pulp, e.g., by a cellulase/hemicellulase treatment.
The prior art does not, however, contain any indication of the use of enzyme treatment in the production of fluff pulp.